The low pressure shaft on a gas turbine engine connects the lower pressure turbine to the fan, and transfers the power from the turbine to the fan. The transferred power is then converted into engine thrust. During engine operation, the shaft experiences very high torsional loads. In the unlikely event of a shaft shear and loss of load, the fuel must be shut off quickly to prevent damage to the engine.
Several methods exist for detecting shaft shear. For example, mechanical axial detection involves using a probe or sensor adjacent to a rear end of the shaft to detect the axial motion of the shaft after the shear. The sheared shaft collides with the sensor, resulting in a fuel shutoff. Another example comprises using a processor to calculate a rate of change of shaft speed. When the rate of change falls below a certain value for a period of time, a fuel shutoff is commanded.
Methods that involve delayed indicators, such as axial displacement and axial movement, or time-consuming signal processing, such as rate of change of shaft speed, are not well-suited for a process requiring rapid fuel shutoff. In addition, methods that require special sensors or additional hardware also have certain disadvantages, such as additional cost and weight, and/or give rise to durability and reliability concerns.
There is therefore a need to improve on existing methods for detecting shaft shear.